Hydrodynamic role of substrate attachment in shaping the suspension-feeding current created by the marine gastropod Crepidula fornicata

As in lamellibranch bivalves, individuals of the common Atlantic slippersnail Crepidula fornicata beat cilia on their gill filaments to produce a suspension-feeding current. Having only one shell and no siphons with which to direct water flow, however, individuals of C. fornicata must adhere to a solid substrate to facilitate normal feeding. Thus, what hydrodynamic role does substrate attachment play in producing, regulating, and directing the suspension-feeding current for this species? Here, a combined particle image velocimetry and computational fluid dynamics study was conducted to address this question. Three findings were obtained: (1) Juveniles of C. fornicata (shell length 6.0–10.6 mm) whose foot was attached to a solid surface generated a strong, fan-like exhalant current and an almost equally strong, convergent inhalant current, both being spatially well extended; (2) juveniles of C. fornicata that were prevented from adhering to any surface also generated a strong, fan-like exhalant current but a much weaker and spatially limited inhalant current; and (3) whether or not they were attached to a solid surface, juveniles of C. fornicata had almost the same performance or system characteristics of the ciliary water pump, including the relationship between flow pressure rise Δp across the ciliary zone and volume flow rate Q, pump resistance Δp/Q, and pressure coefficient for laminar flow C_p,l. These results indicate that the primary hydrodynamic effect of substrate attachment in C. fornicata is to form a complete inhalant chamber with a narrowed opening, such that negative flow pressure develops in the inhalant chamber, and a strong, convergent, spatially well-extended inhalant current is generated to effectively bring in food particles from farther distances and to reduce refiltration of the outflowing water. Finally, ecological trade-offs are discussed regarding the two distinct shell configuration strategies: (1) that of the single-shelled C. fornicata, with only a naturally formed exhalant chamber and opening but not a morphologically defined inhalant chamber and opening, and (2) that of two-shelled bivalves, with naturally formed exhalant and inhalant chambers.